Multimode lasers are employed in various spectroscopic applications, among others. Absorption spectroscopy requires a broad-band emitting light source, when narrow absorption lines are to be studied within the emission spectrum. In the field of highly sensitive spectroscopy, the method of intra-cavity spectroscopy has taken a wide application range in research activities. This purpose requires multimode lasers, the resonators thereof being equipped with a light amplifying medium and, in addition, an absorption cell with a narrow-band absorber.
In recent years, a great number of lasers suited for this type of spectroscopy have been developed. Mostly, these are dye lasers or solid state lasers pumped by an ion laser. Such arrangements may have a smooth emission spectrum since they emit in a plurality of independent laser modes at the same time, yet they are restricted to lab use, because dye lasers are very sensitive to external influences and ion lasers that are the pump source are difficult to transport, have a high energy consumption and must be connected to a cooling water line with a high flow rate. Further, most of the absorbers relevant for practical use have a well measurable absorption in the infrared range, whereas conventional multimode lasers mostly operate best in the visible spectral range.
In this respect, waveguide lasers, and in particular fiber lasers, promise substantial improvements since they are suited for compact designs, are mechanically stable enough and practically insensitive to external influences, and do not require complicated cooling. Moreover, the waveguide may readily be doped with different active ions, whereby the range of emission may be predetermined well and may be set to lie within the infrared range.
However, presently used waveguide lasers are disadvantageous in that they must be pumped by an ion laser. The application of a fiber laser, pumped by an argon ion laser, in intra cavity spectroscopy is known, for example, from a publication in OPTICS LETERS 1993, vol. 18, No. 22, p. 1955, entitled "Intracavity absorption spectroscopy with a Nd.sup.3+ -doped fiber laser". Yet, this arrangement is not suited for mobile use, due to the argon ion laser.
It is further known from the same publication that fiber lasers can be pumped by diode lasers. However, the emission spectra obtained thereby are, as of yet, not suited for absorption spectroscopy with a narrow-band absorber since they show substantial irregularities that make the resolution of the absorption lines impossible.